Heat exchangers which transfer heat to or from a single phase fluid are referred to as single-phase heat exchangers and are used in a variety of applications, ranging from air cooled radiator in automobile to the exotic water-to-ammonia heat exchanger in the living module of the space station. In addition to applications involving the transfer of heat between fluids, single phase gas and liquids are frequently used to remove waste heat from equipment, such as electronic devices, electric motors, internal combustion engines, and power plant condensers. Single phase heat exchangers are used in conjunction with power generation in such applications as pressurized water nuclear reactors and gas-cooled nuclear reactors to remove the energy from the nuclear fuel.
Conventional single-phase heat exchangers cannot achieve high heat fluxes with low pressure drops because of limitations in the heat transfer coefficient. Heat transfer coefficients in conventional single-phase heat exchangers are typically a factor of 10 to 100 less than in boiling or condensing heat exchangers.
Gas cooled heat exchangers are particularly limited in their heat flux capacity because gases have very low heat capacity per unit volume.
The physical size and/or the weight of a heat exchanger can be a burden in some applications. In these applications the heat transferred per unit weight, or per unit volume must be considered when selecting a heat exchanger. Compact heat exchangers are distinguished by having a relatively high mass and/or volume specific capacity when compared to conventional heat exchangers. The book entitled: COMPACT HEAT EXCHANGERS by William M. Kays and A. L. London, McGraw-Hill 1984, is a standard reference for the design and performance analysis of compact heat exchangers. The definitions and terminology used herein will be consistent with those used in COMPACT HEAT EXCHANGERS.
Compact heat exchangers are used in applications where it is desired to reduce the weight and/or volume of the heat exchanger. Compact heat exchangers usually have multiple fluid paths within a confined space to increase the area of the heat exchange surface relative to the volume or the weight of the heat exchanger. Increasing the area by creating multiple fluid paths is frequently achieved by the use of fins.
Alternatively, some compact heat exchangers employ porous heat transfer surfaces to increase the surface area over which the heat transfer fluid passes.
D. B. Tuckerman and R. F. W. Pease in an article entitled "HIGH-PERFORMANCE HEAT SINK FOR VLSI", IEEE ELECTRON DEVICE LETTERS Vol. EDL-2, NO. 5, May 1981, pp 126-29, discuss the use of fins attached to a VLSI substrate. Closely spaced fins are attached to the substrate and fluid is passed by the fins flowing parallel to the substrate. The fins are effective in transferring heat but cause large pressure drops in the fluid.
A number of prior art patents are directed to various aspects of compact heat exchangers. U.S. Pat. No. 3,595,310 of Frederic A. Bernie and Emery I. Valyi, entitled MODULAR UNITS AND USE THEREOF IN HEAT EXCHANGERS, teaches and claims a modular heat exchanger element which comprises a tube which is in turn surrounded by a layer of porous heat conductive material. The '310 patent suggests that the tube be formed of a metal and that the porous tube heat conductive material that surrounds the metal tube be formed of a high heat conductive material such as copper. The patent suggests forming a heat exchanger by combining pieces of the element formed from a soft tube surrounded by a porous material. The porous material is intended to increase the effective surface area in contact with the heat transfer fluid.
U.S. Pat. No. 4,359,181 of John Chisholm, entitled PROCESS FOR MAKING A HIGH HEAT TRANSFER SURFACE COMPOSED OF PERFORATED OR EXPANDED METAL, teaches the process for making a heat transfer surface for a cross flow heat exchanger. The surface is formed by stacking layers of a porous metal lattice or woven material. The stacked material forms volumes having increased surface areas and these volumes are separated by an impervious interface. The flow is in a direction which is substantially parallel to the direction of the impervious interface.
U.S. Pat. No. 4,318,393 of Richard Goldstein, entitled POROUS SURFACE SOLAR ENERGY RECEIVERS, teaches a porous surface receiver for collecting reflected solar radiation. Air is employed as the heat transfer fluid is passed through the porous surface, which is heated by the reflected solar radiation. The patent requires that the air pass through the heat transfer element, does not teach an impervious heat transfer element, and would not be suitable for transferring heat from a solid heat sink.
U.S. Pat. No. 4,494,171 of Timothy J. Bland and Richard E. Niggerman, entitled IMPINGEMENT COOLING APPARATUS FOR HEAT LIBERATING DEVICE, discloses a heat exchanger which uses a series of stacked plates with orifices therein. The plates are parallel to the heat sink which they cool. The orifices generate high velocity jets of the heat transfer fluid which increase the heat transfer, but the orifices restrict the flow which results in large pressure drops as the fluid passes through the heat exchanger.
While the use of fins and/or porous material can increase the heat transfer by increasing the surface area, a large pressure drop in the heat transfer fluid will occur as the heat extracting fluid travels a long torturous path through the heat transfer surface.
Thus there is a need for a heat exchanger which will have high heat flux capability and operate with small pressure drops.